The present invention relates to a method and an apparatus for obtaining information during the operation of a headbox or similar stock-discharging means for a paper machine about the size of a stock discharge slot.
Headboxes are included in the wet end of a paper machine and are used to distribute the stock evenly across the width of the wire and to control the discharge so that this takes place at a uniform velocity and in the same direction across the entire width of the wire. With increasingly higher paper machine speeds, there has been a changeover to closed headboxes which operate under pressure. This has also made it possible to limit the size of the headboxes. While the industry has demanded higher production rates for the paper machines, it also requires that the same high quality standards of the paper be maintained. For special headboxes for two or more stock jets, i.e. so-called multilayer headboxes, the higher production speeds involve major problems in producing a paper web of satisfactory quality with regard to desired uniformity of basis weight and thickness of the combined layers. It is therefore of great importance that the slice opening and the heights of the channels, i.e., the distances between two channel-forming surfaces, be kept constant and under close control so that the discharged stock jets will be uniform with respect to velocity and thickness and so that, as a result, optimum dry weight profiles and/or moisture profiles of the paper web will be obtained. Multilayer headboxes of this type are disclosed for example in U.S. Pat. No. 3,598,696 (Beck); U.S. Pat. No. 3,839,143 (Suckow); U.S. Pat. No. 3,923,593 (Verseput); U.S. Pat. No. 4,021,295 (Schmaeng); and U.S. Pat. No. 4,086,130 (Justus). Headboxes for a single layer web are disclosed, for example, in GB No. 1,488,200 and U.S. Pat. No. 3,994,773 (Wolf et al).
For controlling the velocity of the stock jet emerging from a headbox, it has been heretofore proposed in U.S. Pat. No. 3,487,686 (Salomon) to measure the velocity of the jet emerging from a headbox by generating pressure waves in the jet which are caused to strike two probes arranged at a distance from each other. U.S. Pat. No. 3,341,400 (Grater) discloses a pressure headbox that can be equipped with a piezoelectric crystal for sensing changes in the pressure of the stock flowing through a passage.
According to the methods previously employed for measuring the size of a stock discharge opening, either the movements are measured of an actuator operating a lip member or slice, so that by means of calculation and with a knowledge of the jack rises and lever lengths, the changes in position of a lip member of the headbox can be determined (as in Stotz et al U.S. Pat. No. 3,947,317), or some type of transducer is used to measure the position of the pivoted lip member in relation to some form of reference (as in the aforementioned Wolf et al U.S. Pat. No. 3,994,773). The first method, which is the one most often employed, has the drawback that play in the pivots and deflections of both lip members will not be taken into account. Similarly, the second method fails to record the deflections of the non-pivoted lip member, and thus these vital changes will not be controlled.
In GB No. 1,488,200 a headbox is disclosed that is provided with profile adjusting means comprising a plurality of slice screws and a slice strip deformable by the slice screws and which is secured at its ends to an upper lip and forms a slice aperture together with a lower lip. The slice screws can be provided with e.g. dial indicators of micrometer type or differential transformers in order to measure changes in the slice opening profile. What this and other known systems of slice opening profile adjustment have in common is that the measurements are based on a reference means instead of measuring the distance to an opposed lip member. At present great efforts are being made to produce reliable control systems that are based on the transmission of information from the lip members and the paper web to a computer for more rational and correct decisions concerning necessary adjustments of the slice opening profile.
No method of measuring directly the actual distance between the slice lips and/or partitions continuously during operation, for checking these distances or the slice opening profile, has been suggested previously, despite the fact that there has long been a need for this. Said deflections of the lip members are usually of considerable magnitude, particularly in modern high speed machines with internal pressures in the headbox of up to 5 bar, which on the large areas exposed to this pressure will produce forces acting to separate the lip members in the order of 500,000 N/meter width of the headbox. The value of the deflection of the lip member is usually 2 mm, which should be compared with the slice openings that are normal, 7-12 mm, in other words a considerable deviation.
The need to obtain accurate control of the most vital parts of the headbox is also accentuated in that papermaking now takes place to a large extent under computer-based control and that the need for further computerization, as complete as possible, and thereby the possibility of refined control methods of papermaking has assumed a prominent position in profitability analyses.